diff --git a/darray.h b/darray.h
index 31ab7d4..d6521be 100644
--- a/darray.h
+++ b/darray.h
@@ -161,14 +161,12 @@ rb_darray_free(void *ary)
     xfree(ary);
 }
 
-void ruby_sized_xfree(void *x, size_t size);
-
 static inline void
 rb_darray_free_sized0(void *ary, size_t element_size)
 {
     const rb_darray_meta_t *meta = ary;
     if (meta) {
-        ruby_sized_xfree(ary, sizeof(*meta) + (element_size * meta->capa));
+        ruby_xfree_sized(ary, sizeof(*meta) + (element_size * meta->capa));
     }
 }
 #define rb_darray_free_sized(ary, T) rb_darray_free_sized0((ary), sizeof(T))
@@ -203,7 +201,7 @@ rb_darray_calloc_mul_add_without_gc(size_t x, size_t y, size_t z)
     return ptr;
 }
 
-void *ruby_sized_xrealloc(void *ptr, size_t new_size, size_t old_size);
+void *ruby_xrealloc_sized(void *ptr, size_t new_size, size_t old_size);
 
 /* Internal function. Like rb_xrealloc_mul_add. */
 static inline void *
@@ -212,7 +210,7 @@ rb_darray_realloc_mul_add(void *orig_ptr, size_t capa, size_t element_size, size
     size_t size = rbimpl_size_add_or_raise(rbimpl_size_mul_or_raise(capa, element_size), header_size);
     size_t old_size = (rb_darray_capa(orig_ptr) * element_size) + header_size; // We know it won't overflow
 
-    void *ptr = ruby_sized_xrealloc(orig_ptr, size, old_size);
+    void *ptr = ruby_xrealloc_sized(orig_ptr, size, old_size);
     RUBY_ASSERT(ptr != NULL);
 
     return ptr;
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 1dacd95..3f680e7 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -635,22 +635,29 @@ void rb_gc_impl_set_params(void *objspace_ptr) { }
 
 static VALUE gc_verify_internal_consistency(VALUE self) { return Qnil; }
 
-#define MMTK_HEAP_COUNT 6
-#define MMTK_MAX_OBJ_SIZE 640
-
+#if SIZEOF_VALUE >= 8
+#define MMTK_HEAP_COUNT 12
+#define MMTK_MAX_OBJ_SIZE 1024
 static size_t heap_sizes[MMTK_HEAP_COUNT + 1] = {
-    32, 40, 80, 160, 320, MMTK_MAX_OBJ_SIZE, 0
+    32, 40, 64, 80, 96, 128, 160, 256, 512, 640, 768, MMTK_MAX_OBJ_SIZE, 0
 };
+#else
+#define MMTK_HEAP_COUNT 5
+#define MMTK_MAX_OBJ_SIZE 512
+static size_t heap_sizes[MMTK_HEAP_COUNT + 1] = {
+    32, 64, 128, 256, MMTK_MAX_OBJ_SIZE, 0
+};
+#endif
 
 void
 rb_gc_impl_init(void)
 {
     VALUE gc_constants = rb_hash_new();
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(SIZEOF_VALUE >= 8 ? 64 : 32));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RBASIC_SIZE")), SIZET2NUM(sizeof(struct RBasic)));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), INT2NUM(0));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVARGC_MAX_ALLOCATE_SIZE")), LONG2FIX(MMTK_MAX_OBJ_SIZE));
-    // Pretend we have 5 size pools
-    rb_hash_aset(gc_constants, ID2SYM(rb_intern("SIZE_POOL_COUNT")), LONG2FIX(MMTK_HEAP_COUNT));
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_COUNT")), LONG2FIX(MMTK_HEAP_COUNT));
     // TODO: correctly set RVALUE_OLD_AGE when we have generational GC support
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OLD_AGE")), INT2FIX(0));
     OBJ_FREEZE(gc_constants);
diff --git a/test/.excludes-mmtk/TestObjSpace.rb b/test/.excludes-mmtk/TestObjSpace.rb
index 82858b2..94eb2c4 100644
--- a/test/.excludes-mmtk/TestObjSpace.rb
+++ b/test/.excludes-mmtk/TestObjSpace.rb
@@ -1,5 +1,4 @@
 exclude(:test_dump_all_full, "testing behaviour specific to default GC")
 exclude(:test_dump_flag_age, "testing behaviour specific to default GC")
 exclude(:test_dump_flags, "testing behaviour specific to default GC")
-exclude(:test_dump_includes_slot_size, "can be removed when pool 0 slot size is 32 bytes")
 exclude(:test_dump_objects_dumps_page_slot_sizes, "testing behaviour specific to default GC")